Anthropogenic perturbation enhances the release of geogenic Mn to groundwater: Evidence from hydrogeochemical characteristics

High geogenic Mn groundwater is widespread around the world and has also proved to be harmful to human health, especially to the IQ of Children. The natural release of Mn from aquifer sediments in slightly reducing condition is believed to be the primary cause. However, there isn't enough evidence to prove that anthropogenic activities promote the reductive release of Mn. Here a Historical Petrochemical Waste Storage Site (HPWSS) was studied to evaluate its impact on groundwater quality. Significantly elevated Mn, as well as elevated TDS, anionic surfactants, and organic pollutants, were found in the shallow aquifer (9-15 m) groundwater compared to the surrounding area. The Mn was believed to be generated in-situ, while others are caused by anthropogenic pollution. The good correlations between Mn and NH4+, HCO3−, I, As, Co, V, Ti, respectively, showed the Mn mobilization was mainly attributed to the reductive dissolution of Mn oxides/hydroxides. The potential processes leading to this enhanced Mn release are discussed, including 1) the infiltration of high salinity water which solubilized sediment organic matter (OM); 2) the anionic surfactants that promoted the dissolution and mobilization of surface-derived organic pollutants as well as sediment OM. Any of these processes may have provided a C source to stimulate the microbial reduction of Mn oxides/hydroxides. This study showed the input of pollutants could change the redox and dissolution conditions of the vadose zone and aquifer, causing a secondary geogenic pollution risk in groundwater. Since Mn is easily mobilized in suboxic condition as well as its toxicity, the enhanced release due to anthropogenic perturbation merits more attention.